Three Roads To Quantum Gravity

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THE SOUND OF SPACE IS A STRING 153 time space B Electron moving from point A to B A Path in spacetime of a photon An electron absorbs a photon and changes its trajectory FIGURE 33 The basic processes in the theory of electrons and photons (called quantum electrodynamics, or QED for short.) Electrons and photons can move freely in spacetime, or they can interact in events in which an electron absorbs or emits a photon. In the language of these diagrams, now known appropriately as Feynman diagrams, it is very easy to explain what string theory is. The basic postulate of the theory is that there are no particles, only strings moving in space. A string is just a loop drawn in space. It is not made of anything, just as a particle is thought of as a point and nothing else. There is only one kind of string, and the different kinds of particle are postulated to be nothing but different modes of vibration of these loops. So, as shown in Figure 35, photons and electrons are to be thought of just as different ways in which a string can vibrate. When a string moves in time it makes a tube rather than a line (Figure 35). Two strings can also join and merge into one (Figure 36), or one string can split into two. All the interactions that occur in nature, including those of photons and electrons, can be interpreted in terms of these splittings and joinings. We can see from these pictures that string theory gives a very satisfactory uni®cation and simpli®cation of the physical processes represented in Feynman diagrams. Its main virtue is that it gives a simple way of ®nding theories that make consistent physical predictions. The trouble with Feynman's method is that it always leads
154 THREE ROADS TO QUANTUM GRAVITY space time FIGURE 34 The processes illustrated in Figure 33 are put together to make Feynman diagrams, which are pictures of the possible ways a process can happen. Shown here are some of the ways in which two electrons can interact simply by absorbing and emitting photons. Each one is a story that is a possible piece of the history of a universe.
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To my parents Pauline and Michael C
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I POINTS OF DEPARTURE
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CHAPTER 2 .........................
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II WHAT WE HAVE LEARNED
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CHAPTER 7 .........................
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Page 115 and 116: CHAPTER 9 .........................
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Page 180 and 181: THE HOLOGRAPHIC PRINCIPLE 171 gener
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Page 190 and 191: HOW TO WEAVE A STRING 181 thing!" I
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Page 194 and 195: HOW TO WEAVE A STRING 185 What is r
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WHAT CHOOSES THE LAWS OF NATURE? 20
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WHAT CHOOSES THE LAWS OF NATURE? 20
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EPILOGUE: .........................
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EPILOGUE: 209 And quantum gravity i
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EPILOGUE: 211 of systems containing
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POSTSCRIPT 213 These cosmic rays ar
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POSTSCRIPT 215 Three explanations h
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POSTSCRIPT 217 light, which is the
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POSTSCRIPT 219 This is good news in
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POSTSCRIPT 221 But there is a secon
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POSTSCRIPT 223 Chopin Soo and Marti
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POSTSCRIPT 225 speed of light with
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GLOSSARY 227 cannot be greater than
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GLOSSARY 229 event In relativity th
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GLOSSARY 231 Newton's gravitational
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GLOSSARY 233 spin The angular momen
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SUGGESTIONS FOR FURTHER READING ...
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SUGGESTIONS FOR FURTHER READING 237
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SUGGESTIONS FOR FURTHER READING 239
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INDEX .............................
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INDEX 243 black hole, 70, 73±4, 17
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INDEX 245 black hole formation, 189
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Three Roads To Quantum Gravity

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